The US president’s pleas for harmony came as officials scrambled to find the perpetrator of the thwarted bomb attacks against former President Barack Obama, Hillary Clinton, CNN and others.

The scripted message was a dissonant one for the president, who has repeatedly blasted his political opponents as criminals and argued that they will destroy the country if they win control of Congress in the mid-terms.

Mr Trump has frequently labelled stories he doesn’t like as “fake news” and many reporters as “enemies of the people.” He has also denounced his political enemies in deeply personal terms and even described those who tried to thwart his second Supreme Court nominee as “evil.

He said: “The media also has a responsibly to set a civil tone and to stop the endless hostility and constant negative and oftentimes false attacks and stories.”

“We want all sides to come together in peace and harmony,” Mr Trump said at a campaign rally in Wisconsin. “Any acts or threats of political violence are an attack on our democracy itself.”

The president noted the unusually subdued tone of his remarks.

“By the way, do you see how nice I’m behaving tonight?” he said. “Have you ever seen this?”

Law enforcement authorities have given no motive for the mail bombs. But they have prompted immediate debate over whether increasingly personal and hard-edged rhetoric has contributed to a potentially dangerous political climate.

Trump critics have blamed him for the tone. The president did not take any responsibility.

Mr Trump spoke to thousands of supporters in a central Wisconsin rally as he looked to boost struggling Republican candidates less than two weeks out from the midterm elections. He visited the tiny city of Mosinee, which has a population of 4,000. He won that part of the state by double digits in 2016.

Republicans in the state have been growing increasingly nervous about the prospects of holding onto the governor’s office, held by Scott Walker, let alone state Sen. Leah Vukmir’s chances of picking up a US Senate seat held by a well-positioned Democratic incumbent.

Mindful of the news, Mr Trump refrained from delivering some of the most pointed riffs of his regular routine, including steering clear of two of his favourite targets: Mrs Clinton and California Rep. Maxine Waters, whose office appears to have been a target of one of the mail explosives.

Mr Trump also skipped his usual litany of insults against his potential 2020 rivals, including former Vice President Joe Biden, whom he often jokes he would be able to easily beat in a fistfight.

But he had harsh words for Democratic Sen. Tammy Baldwin, branding her Vukmir’s “radical far-left opponent” who “wants a socialist takeover of health care.”

“You know I’m trying to say that very nicely,” he told the crowd with a smile. “Normally I’d scream, ‘They want a socialist takeover!'” he said, mimicking his own bellicose delivery. “Now I say, ‘Wants a socialist takeover,'” he said, feigning a professorial tone. “I’m trying to be nice.”

And even though Mr Trump refrained from mentioning Mrs Clinton, his crowd couldn’t resist. They burst into a “Lock her up!” chant during the preshow before he arrived.